Tides, and the pull of the moon and sun | Earth | EarthSky Tides, and the pull of the moon and sun By Deborah Byrd in Earth | April 7, 2016 The sun, the moon, the shape of a beach and larger coastline, the angle of a seabed leading up to land, and the prevailing ocean currents and winds all affect the height of the tides. This beautiful image is from EarthSky Facebook friend John Lloyd Griffith We all know the moon is primarily responsible for the rising and falling of ocean tides. In most places, but not everywhere, there are two high tides and two low tides a day. For any particular spot on Earth’s surface, the height of the tides and their fluctuation in time depends not only on the moon, but also on the sun – and also on the shape of the specific beach, the larger coastline, the angle of the seabed leading up to land, and the prevailing ocean currents and winds. The difference in height between high and low waters varies as the moon waxes and wanes from new to full and back to new again. The larger tides are called spring tides (nothing to do with season of spring).

The smaller tides are called neap tides. Click the links below to learn more about the tides. Why are there two high tides and two low tides each day? Around each new moon and full moon – when the sun, Earth, and moon are located more or less on a line in space – the range between high and low tides is greatest. These are called spring tides. Image via physicalgeography.net What are spring tides? Around each new moon and full moon, the sun, Earth, and moon arrange themselves more or less along a line in space. Then the pull on the tides increases, because the gravity of the sun reinforces the moon’s gravity. In fact, the height of the average solar tide is about 50% the average lunar tide. Thus, at new moon or full moon, the tide’s range is at its maximum. This is the spring tide: the highest (and lowest) tide. Spring tides are not named for the season. This is spring in the sense of jump, burst forth, rise. So spring tides bring the most extreme high and low tides every month, and they happen around full and new moon. What are perigean spring tides?

When the new moon or full moon closely aligns with perigee – closest point to Earth in the moon’s orbit – then we have an extra-large perigean spring tides . For example, in the year 2016, the April 7 new moon will closely align with perigee and the November 14 full moon will closely coincide with perigee, to bring forth perigean spring tides. When is the next supermoon? It’s when a spring tide coincides with a time of heavy winds and rain – flooding due to a weather extreme – that the most extreme flooding occurs. Around each first quarter moon and last quarter moon – when the sun and moon are at a right angle to Earth – the range between high and low tides is least. These are called neap tides. Image via physicalgeography.net What are neap tides? These sorts of tides occur halfway between each new and full moon – at the first quarter and last quarter moon phase – when the sun and moon are at right angles as seen from Earth. Then the sun’s gravity is working against the gravity of the moon, as the moon pulls on the sea.

This is the neap tide: the tide’s range is at its minimum. Neap tides happen approximately twice a month, once around first quarter moon and once around last quarter moon. There’s about a seven-day interval between spring tides and neap tides. Earth has two tidal bulges, one of the side of Earth nearest the moon (where the moon’s gravity pulls hardest), and the other on the side of Earth farthest from the moon (where the moon’s gravity pulls least). Why are there two high tides and two low tides each day? If the moon is primarily responsible for the tides, why are there two high tides and two low tides each day in most places, for example, the U.S. eastern seaboard? It seems as if there should just be one. If you picture the part of Earth closest to the moon, it’s easy to see that the ocean is drawn toward the moon. That’s because gravity depends in part on how close two objects are. But then why – on the opposite side of Earth – is there another tidal bulge, in the direction opposite the moon?

It seems counterintuitive, until you realize that this second bulge happens at the part of Earth where the moon’s gravity is pulling the least. Looking for a tide almanac? EarthSky recommends … Earth spins once every 24 hours. So a given location on Earth will pass “through” both bulges of water each day. Of course, the bulges don’t stay fixed in time. They move at the slow rate of about 13.1 degrees per day – the same rate as the monthly motion of the moon relative to the stars. Other factors, including the shape of coastlines, etc., also influence the time of the tides, which is why people who live near coastlines like to have a good tide almanac . The Moon And Tides Moon Tides How The Moon Affects Ocean Tides... The word "tides" is a generic term used to define the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the sun. To a much smaller extent, tides also occur in large lakes, the atmosphere, and within the solid crust of the earth, acted upon by these same gravitational forces of the moon and sun.

What are Lunar Tides Tides are created because the Earth and the moon are attracted to each other, just like magnets are attracted to each other. The moon tries to pull at anything on the Earth to bring it closer. But, the Earth is able to hold onto everything except the water. Since the water is always moving, the Earth cannot hold onto it, and the moon is able to pull at it. Each day, there are two high tides and two low tides. The ocean is constantly moving from high tide to low tide, and then back to high tide. There is about 12 hours and 25 minutes between the two high tides. Tides are the periodic rise and falling of large bodies of water. Winds and currents move the surface water causing waves. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Ocean levels fluctuate daily as the sun, moon and earth interact.

As the moon travels around the earth and as they, together, travel around the sun, the combined gravitational forces cause the world's oceans to rise and fall. Since the earth is rotating while this is happening, two tides occur each day. What are the different types of Tides When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season. When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides. Spring Tides When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.

Neap Tides During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons. The Proxigean Spring Tide is a rare, unusually high tide. This very high tide occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee) and in the New Moon phase (when the Moon is between the Sun and the Earth). The proxigean spring tide occurs at most once every 1.5 years. High Tide / Low Tide Examples A view of the tides at Halls Harbour on Nova Scotia's Bay of Fundy. This is a time lapse of the tidal rise and fall over a period of six and a half hours. During the next six hours of ebb the fishermen unload their boats on the dock. That's a high tide every 12 and 1/2 hours!

There are two high tides every 25 hours. A Few Facts About Lunar Tides The gravitational force of the moon is one ten-millionth that of earth, but when you combine other forces such as the earth's centrifugal force created by its spin, you get tides. The sun's gravitational force on the earth is only 46 percent that of the moon. Making the moon the single most important factor for the creation of tides. The sun's gravity also produces tides. But since the forces are smaller, as compared to the moon, the effects are greatly decreased. Tides are not caused by the direct pull of the moon's gravity. The moon is pulling upwards on the water while the earth is pulling downward. Slight advantage to the moon and thus we have tides. Whenever the Moon, Earth and Sun are aligned, the gravitational pull of the sun adds to that of the moon causing maximum tides. Spring tides happen when the sun and moon are on the same side of the earth (New Moon) or when the sun and moon are on opposite sides of the earth (Full Moon).

When the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides. Spring tides and neap tide levels are about 20% higher or lower than average. Offshore, in the deep ocean, the difference in tides is usually less than 1.6 feet The surf grows when it approaches a beach, and the tide increases. In bays and estuaries, this effect is amplified. (In the Bay of Fundy, tides have a range of 44.6 ft.) The highest tides in the world are at the Bay of Fundy in Nova Scotia, Canada. Because the earth rotates on its axis the moon completes one orbit in our sky every 25 hours (Not to be confused with moon's 27 day orbit around the earth), we get two tidal peaks as well as two tidal troughs. These events are separated by about 12 hours. Since the moon moves around the Earth, it is not always in the same place at the same time each day.

So, each day, the times for high and low tides change by 50 minutes. The type of gravitational force that causes tides is know as "Tractive" force. FAQs About Lunar Tides From - "The Astronomy Cafe" Why are there no ocean tides at the equator? "Tides are a very complex phenomenon. For any particular location, their height and fluctuation in time depends to varying degrees on the location of the Sun and the Moon, and to the details of the shape of the beach, coastline, coastline depth and prevailing ocean currents. The tidal bulge of the Moon follows along the path on the earth's surface which intersects with the orbital plane of the Moon. This plane is tilted about 23 degrees with respect to the equatorial plane of the earth. The result is that near the equator, the difference between high tide and low tide is actually rather small, compared to other latitudes. To see this, draw a circle inscribed in an ellipse, with the major axis of the ellipse rotated by 23 degrees with respect to the circle's horizontal diameter. Now measure the height of the elliptical contour just above the 'equator' of the circle.

You will see that it is quite small compared to other positions on earth, particularly at latitudes of 23 degrees or so. Even larger differences can occur depending on the shape of a bay or inlet or continental shelf." - Dr. Odenwald's ASK THE ASTRONOMER Why are ocean tides so different everywhere? "Because they depend on many factors including the geometry of your local coastline, and exactly where the Sun and Moon are located. Also, like the surface of a vibrating drum, the world oceans have vibratory modes that get stimulated in changing ways from minute to minute. Finally, there are storms at sea and elsewhere which move large quantities of water. Detailed forecasts are available for high and low tides in all sea ports." - Dr. Odenwald's ASK THE ASTRONOMER Why aren't the Atlantic and Pacific coast tides the same? "The nature of tides on the Earth's oceans is very complex. The oceans are, of course, being periodically 'forced' by a number of tidal sources including the Moon and the Sun, but this forcing has a number of different periods and harmonics.

The two dominant periods are sue to the Sun and Moon, these are referred to as the S1 and M2 'modes' which have roughly 12 hour periods because they raise TWO water tides on the ocean diametrically opposite each other. But, for a variety of reasons, any given port will not have two high and two low tides each day; also called 'semi-diurnal tides'. A careful monitoring of the tides at any port for several years will show that in addition to the major modes, there are as many as 300 minor or 'harmonic' modes as well. The World Ocean is a complex dynamical system. The natural velocity of a water disturbance depends on the depth and salinity of the water at each point it passes. When bodies of land circumscribe bodies of water, they produce a collection of resonating systems that favor water oscillations with certain frequencies over others. From among the 300+ harmonics that can be measured, every port and coastal location has its own unique signature depending on its latitude, longitude, water depth and salinity.

The result is that the 'two high two low' tide rule can be strongly modified so that the time between successive high tides can be greater than or less that 12 hours in many cases. The result is that for some locations, there can be days when only one high tide occurs. Looking at the Atlantic and Pacific Coast tide tables for 1995, the data for the various 'Standard Ports' showed that virtually all days had two high tides and two low tides in San Diego, San Francisco, New York and Charleston. There were, however a few days every few months when only a single high tide occurred." - Dr. Odenwald's ASK THE ASTRONOMER What is a Proxigean Spring Tide? "The Moon follows an elliptical path around the Earth which has a perigee distance of 356,400 kilometers, which is about 92.7 percent of its mean distance. Because tidal forces vary as the third power of distance, this little 8 percent change translates into 25 percent increase in the tide- producing ability of the Moon upon the Earth.

If the lunar perigee occurs when the Moon is between the Sun and the Earth, it produces unusually high Spring high tides. When it occurs on the opposite side from the Earth that where the Sun is located ( during full moon) it produces unusually low, Neap Tides. The High, High Tide is called the Proxigean Spring Tide and it occurs not more than once every 1.5 years. Some occurrences are more favorable that others. A very interesting book "Tidal Dynamics" by Fergus J. Wood, published in 1986 by Reidel Publishing Company, talks at great length about these tides, and their environmental consequences. Because of the gravitational nature of the interaction between the Earth, the Moon, and the water on the Earth, there is a curious amplification event called 'evection' that occurs when the Moon is at its closest 'perigee' distance called its 'proxigee'. The Moon draws even closer to the Earth than its ordinary perigee distance. Because of the complex dynamics of the Earth's oceans, their inertia, friction with the ocean floor, internal viscosity and the distribution of the continents, the maximum tides do not always coincide with the optimal times of proxigee.

Still, these tides can produce enormous damage when all factors come together optimally. There are many recorded instances of unusually high storm or coastal flooding during the proxigean times. On January 9, 1974 the Los Angeles Times reported 'Giant Waves Pound Southland Coast". During the last 400 years, there have been 39 instances or 'Extreme Proxigean Spring Tides' where the tide-producing severity has been near the theoretical maximum. The last one of these was on March 7 1995 at 22:00 hours Greenwich Civil Time during a lunar Full Moon. There were, in fact cases of extreme tidal flooding recorded during these particular spring tides which occur once every 31 years." - Dr. Odenwald's ASK THE ASTRONOMER If the Moon were to escape, what would happen to the Earth's oceans? "What happens is that the lunar water tides on the Earth go away, but the solar water tides still occur, but with about 1/3 or so the amplitude. There are still daily high and low tides, but they would be noticeably smaller.

There would be no 'Spring' or 'Neap' tides, however."- Dr. Odenwald's ASK THE ASTRONOMER Why does the Moon produce TWO water tides on the Earth and not just one? "It is intuitively easy to understand why the gravitational pull of the Moon should produce a water tide on the Earth in the part of the ocean closest to the moon along the line connecting the center of the Moon with the center of the Earth. But in fact not one but TWO water tides are produced under which the Earth rotates every day to produce about two high tides and two low tides every day. How come? It is not the gravitational force that is doing it, but the change in the gravitational force across the body of the Earth. If you were to plot the pattern of the Moon's 'tidal' gravitational force added to the Earth's own gravitational force, at the Earth's surface, you would be able to resolve the force vectors at different latitudes and longitudes into a radial component directed towards the Earth's center, and a component tangential to the Earth's surface.

On the side nearest the moon, the 'differential' gravitational force is directed toward the Moon showing that for particles on the Earth's surface, they are being tugged slightly towards the Moon because the force of the Moon is slightly stronger at the Earth's surface than at the Earth's center which is an additional 6300 kilometers from the Moon. On the far side of the Earth, the Moon is tugging on the center of the Earth slightly stronger than it is on the far surface, so the resultant force vector is directed away from the Earth's center. The net result of this is that the Earth gets deformed into a slightly squashed, ellipsoidal shape due to these tidal forces. This happens because if we resolve the tidal forces at each point on the Earth into a local vertical and horizontal component, the horizontal components are not zero, and are directed towards the two points along the line connecting the Earth and the Moon's centers. These horizontal forces cause rock and water to feel a gravitational force which results in the flow of rock and water into the 'tidal bulges'. There will be exactly two of these bulges.

At exactly the positions of the tidal bulges where the Moon is at the zenith and at the nadir positions, there are no horizontal tidal forces and the flow stops. The water gets piled up, and the only effect is to slightly lower the weight of the water along the vertical direction. Another way of thinking about this is that the gravitational force of the Moon causes the Earth to accelerate slightly towards the Moon causing the water to get pulled towards the Moon faster than the solid rock on the side nearest the Moon. On the far side, the solid Earth 'leaves behind' some of the water which is not as strongly accelerated towards the Moon as the Earth is. This produces the bulge on the 'back side' of the Earth."- Dr. Odenwald's ASK THE ASTRONOMER What Causes Tides? "There are several kinds of tides. The ones that break upon a beach every 10 seconds to a minute are caused by sea level disturbances out in the ocean produced by such things as storms. Also, the various circulation currents of sea water can have velocity components directed towards the land which will bring water up onto the beach.

As this water travels towards the beach from deep water to shallow water, its amplitude will increase until it finally 'breaks' as a full-fledged breaker, suitable for surfing etc. Now, underlying this minute to minute activity is a slower water wave which causes an alternating pattern of high-tide, low-tide, high-tide, low-tide in most places on the Earth that are directly on the ocean. This roughly 6 hour cycle is caused by the gravitational tugging of the Moon upon the Earth. This 'tidal' pull causes the shape of the solid Earth to be not perfectly round by something like a few dozen yards over its entire 27,000 mile circumference. The Earth gets distorted a small bit, but because it is solid rock its a small effect. The water in the oceans, however, gets distorted into a roughly ellipsoidal ( football-like) shape with a much larger amplitude. The orientation of this shape changes from minute to minute as the Moon orbits the Earth, which is why the high and low tide times change all the time. The Moon causes these tides by deforming the oceans, and as the Earth rotates under this ocean bulge, it causes a high tide to propagate onto beaches.

Because there are two bulges, we get two high tides, and also two low tides each day. The Sun also causes tides on the Earth because even though it is so far away, it is very massive. These solar tides are about half as strong as the ones produced by the Moon, and they cause the so-called Spring tides and the Neap Tides. When the bulge of ocean water raised by the Moon is added the a similar tidal bulge raised by the Sun, you get a higher, high tide called the Spring Tide. When the solar low tide is added to the lunar low tide, you get the Neap Tide. There may be even weaker tides caused by the gravitational influences of the planets Mars and Venus, but they are probably lost in the daily noise of individual tides."- Dr. Odenwald's ASK THE ASTRONOMER When the Earth, Moon and Sun are aligned for Spring Tides, are they highest at Full or New Moon? "Spring tides are about the same height whether at New or Full Moon, because the tidal bulge occurs on both sides of the Earth...the side toward the Moon ( or sun) and the side away from the Moon (or Sun).

They will not be equally high because the distance between the Earth and Sun, and the Earth and Moon both vary and so will their tide producing effectiveness. The highest Spring tides occur when the Moon is at its closest to the Earth...the so-called Perigee Tide."- Dr. Odenwald's ASK THE ASTRONOMER Tides explained Yet More... Tides Most people grow up thinking that the tides are caused by the moon, and indeed that gravitational 'pull' of the moon is a major factor, as is the gravitational effect of the sun but there is another major factor, which is less often mentioned, and that is the force created by the rotation of the earth itself. So the diagram on the left shows how the water on the earth gets pulled into a bulge one way by the moon's gravity and into a bulge on the opposite side by the rotational force of the earth.

As the earth rotates on its axis once every 24 hours, if the moon stood still then the moon will be overhead and 'underfoot' once in every 24 hours giving (in most places) 2 high and 2 low tides a day, but as the moon is also orbiting in the same direction as the earth revolves it actually takes about 24 hours and 50 minutes for the moon to be in the same place overhead. So the period from one high tide to the next is about 12 hours 25 minutes. Spring Tides When the sun, moon and earth all line up at new (as in the picture) or full moon then we get the highest (and indeed lowest) tides which are called 'spring' tides (nothing to do with the time of year!). In fact it takes a bit of time for the enormous mass of water to move, so the spring tide will actually occur a couple of days after the new (or full) moon, so the picture on the left is a simplification.

You can find more on ( moon phases here ) Because of the tilt of the earth on its axis, at the equinox the sun appears to be over the equator and if there is a new or full moon at about the same time then all the forces contrive to create the highest of high tides and if the moon is also at perigee (closest to the earth in its orbit) then that will produce the biggest tides of all. Neap Tides When the moon is at first or third quarter (you can find more on moon phases here ) then the moon and sun are exerting forces from two different directions and the overall effect on the water is less, so the high tides are lower than average and the low tides are higher than the average and we call these low tides 'neap' tides. The word 'neap' is thought to have originated from the Middle English Word 'neep' meaning small. Other Factors and Facts Although the sun, the moon and the rotation of the earth are the major forces involved in creating the tides the local conditions such as the shoreline and the contour of the ocean floor also have an effect.

Because of this not everywhere has 2 tides a day - there are some places that experience what is known as a double-high water (e.g. Southampton) or double-low water (e.g. Portland). The highest tides of all (17m) occur in Canada and after a long running dispute between the famous tides of the Bay of Fundy and those of Ungava Bay on the northern coast of Quebec, the Canadian Hydrographic Service has declared a draw. Most people think of the Mediterranean area as being without tides and it is true that the tides there are very small, and can often go unnoticed as atmospheric conditions and headwinds can attenuate the effect of these tides making them difficult to see. The very smallest tidal ranges (actually 0) are found in the Adriatic and South of Sicily. "Everything has a natural explanation. The moon is not a god, but a great rock, and the sun a hot rock. " Anaxagoras "I think that when NASA works on a moon shot, they know too well that all of the people working on it must do their job at 110 percent.

Sometimes they probably put in 18 hour days, but they're aiming for the moon, and that's what counts. " Marvin Hamlisch What are spring and neap tides? Home Ocean Facts What are spring and neap tides? What are spring and neap tides? A spring tide—popularly known as a "King Tide"—refers to the 'springing forth' of the tide during new and full moon. A neap tide—seven days after a spring tide—refers to a period of moderate tides when the sun and moon are at right angles to each other. A spring tide is a common historical term that has nothing to do with the season of spring. Rather, the term is derived from the concept of the tide "springing forth." Spring tides occur twice each lunar month all year long without regard to the season. Neap tides, which also occur twice a month, happen when the sun and moon are at right angles to each other. Tides are long-period waves that roll around the planet as the ocean is "pulled" back and forth by the gravitational pull of the moon and the sun as these bodies interact with the Earth in their monthly and yearly orbits.

During full or new moons—which occur when the Earth, sun, and moon are nearly in alignment—average tidal ranges are slightly larger. This occurs twice each month. The moon appears new (dark) when it is directly between the Earth and the sun. The moon appears full when the Earth is between the moon and the sun. In both cases, the gravitational pull of the sun is "added" to the gravitational pull of the moon on Earth, causing the oceans to bulge a bit more than usual. This means that high tides are a little higher and low tides are a little lower than average. These are called spring tides, a common historical term that has nothing to do with the season of spring. Rather, the term is derived from the concept of the tide "springing forth." Spring tides occur twice each lunar month all year long, without regard to the season. Seven days after a spring tide, the sun and moon are at right angles to each other. When this happens, the bulge of the ocean caused by the sun partially cancels out the bulge of the ocean caused by the moon.

This produces moderate tides known as neap tides, meaning that high tides are a little lower and low tides are a little higher than average. Neap tides occur during the first and third quarter moon, when the moon appears "half full." NOAA’s tide and tidal current predictions take into account astronomical considerations due to the position of the moon and the sun. Search Our Facts UCSB Science Line UCSB Science Line How does the Moon control the tides? Answer 1: Although it is often asserted that the moon "controls" the tides, this is really an oversimplification of the tidal system. In fact there are many factors which determine the tides, including the moon, the sun, the rotation of the earth, the geomorphology of the ocean basin, and the location of the particular spot where you're measuring the tide along that basin. All of these factors interact in a complex way to determine the specifics of the tide's characteristics at each location on Earth. However, it is true that the basic global phenomenon of tides (ignoring local particulars and peculiarities) can be explained as a consequence of rotation and gravity, with the moon being the factor that tips the rotation/gravity system into one which produces tides.

Before you can understand how the moon's influence helps to create tides, you must first understand a little about the properties of water. Water molecules are attracted to each other and tend to stick together. (This is the reason why two droplets of water merge quickly into one when they touch each other, and why a single drop of water will sit as a little ball on the table rather than spreading out into a thin film). This property of water is called "cohesion". Because water is cohesive, it tends to act together like a single body or mass. You can think of it kind of like a weak blob of Jello: when you push or pull on one part, the rest of it moves too. As the earth rotates, it is subjected to a centrifugal force which pulls the ocean water molecules away from the earth's surface. (This is just like when you spin around very fast, and your arms want to fly away from your body). Because the ocean water molecules stick together, this caused the entire ocean to bulge outward in the center, creating a kind of "mound" of water encircling the earth. Because of this, the world's oceans bulge around the equator, and get thinner toward the poles.

(This means that the surface of the ocean is higher in the middle than on the northern and southern edges!) If the earth simply rotated on its own axis, then even though there would be this bulge there would be no tide. However, because the moon is very close to the earth, its gravity exerts an effect on the rotational system of the earth, causing it to also rotates in tandem with the moon on a different axis. This rotation is like the rotation of a baton, with the earth and moon each representing on of the balls on the end of the baton. Because of this additional rotation, there is an additional centrifugal force exerted on the side of the earth that is away from the moon (on the "outside" of the two baton balls). Thus, the centrifugal force is greatest on the side of the earth that is away from the moon, and there is a greater bulge in the water on that side. Now, remember that the earth also rotates on its own axis. Say that you are sitting at the beach in Santa Barbara watching the water level for the whole day.

As the rotation of the earth carries California away from the moon, you would see the water begin to rise, because the spot you are watching is moving into the part where the ocean is bulging; as your beach moves into this bulge of rising water, it gets flooded a little bit, and you might have to move your blanket higher up to avoid getting it wet! This explains why there is a high tide once a day. However, you have probably learned that there are usually two high tides every 24 hours. Where does the second one come from? The answer to that question is also that it is a consequence of the moon's gravity affecting the earth, but in a different way. Whereas the first high tide is due to the fact that the moon and earth are linked into a rotating tandem by their gravity, the second high tide is a result of the moon's direct gravitational pull on the earth's oceans. When a region of the ocean is directly under the moon, the moon's gravity pulls the ocean away from the earth, increasing the size of the water bulge there. This is the second high tide!

Thus, there are actually two bulges in the oceans on the earth: one bulge on the same side as the moon (caused by the moon's gravitational pull), and the other on the opposite side (caused by the centrifugal force of the earth-moon tandem rotation). As California passes under the moon, the water along the California coast is pulled into a bulge, causing a high tide. As California moves away from the moon, the gravitational pull gets less and the water starts to go back down. However, as California passes to the opposite side of the earth the water starts getting pulled up onto shore again by the centrifugal force, causing a second high tide, and then as California moves away from this spot it goes down again, for the second low tide. So, as you can see, it is clearly due to the moon that we have our tides. However, tides vary all over the world. Answer 2: Tides are created by the moon (and to some degree actually, the sun) because of the moon's gravity and the rotation of the earth. The moon has a large mass that exerts a large gravitational force or 'pull' on the earth.

However, because of the shape and size of the earth, the portion of the earth's oceans that are facing the moon at any time are closer to the moon than the center of the earth, and therefore will feel a greater pull, and will be pulled outward towards the moon forming a bulge, or high tide. Likewise, the portion of the earth's oceans that are on the opposite side of the earth from the Moon are farther away from the moon than the earth's center, and the center of the earth will feel the greater pull and be pulled away from the ocean on the side opposite the moon. This forms a second bulge or high tide on the opposite side of the earth. As the earth rotates on its axis, the tides will move on the surface of the earth so that the high tide remains fixed on the side directly facing, and directly opposite the moon. Many other factors can affect the path taken by tides in each ocean basin, such as the shape of surrounding landmasses, but the dominant effect is from the moon's gravitational pull. Answer 3: The tides are generated by the gravitational pull of the moon on the Earth.

It is tempting to think that the moon pulls on the ocean, which makes it rise slightly - and this rising of the ocean is interpreted as a high tide by folks on the Earth. In this picture, as the Earth rotates under the moon, the high tide would appear to move - and you would see a high tide every 24 hours. It turns out, however, that high tide occurs every 12 hours! So this theory of the tides is definitely wrong. The way this really works is a little tricky. It is helpful to try to draw a picture of what I will try to describe for you. It might also help to go get a book on the solar system that has a picture of this drawn for you so you can follow along. It's not that it's too hard to understand how the tides work; it's just that what is happening is very geometrical and hard to describe without pictures. So if you don't understand, it is definitely my fault. You probably heard that the gravitational force gets weaker as you get farther away. This means that, things close to the moon get pulled with a higher force than things farther away from the moon.

It is helpful here grab a piece of paper and draw a circle to represent the Earth, inside a larger circle that represents the surface of the ocean. Now next to those two concentric circles, draw a smaller circle to represent the moon. As you can see from this diagram, the part of the ocean closest to the moon gets pulled with a stronger force than the Earth, which gets pulled with an even stronger force than the part of the ocean farther away from the moon. If the gravitational force causes things to shift so that they are closer to the moon, the part of the ocean closest to the moon gets pulled farther than the earth which gets pulled farther than the far side of the ocean. The result is that the oceans look like they have been pulled away from the Earth on BOTH the side closest to the moon and the side farthest from the moon, implying that high tide occurs on opposite ends of the Earth at the same time. Now, from the point of view of people on the Earth, the high tide appears to move as the Earth rotates, with a high tide occurring every 12 hours. As often happens in science, the details are a little more complicated. The basic picture, however, is correct.

Answer 4: The tides are due to the gravitational pull of the Moon and the Sun on the Earth. Because the gravitational force decreases as the distance squared, the part of the Earth that is closer to the Moon or the Sun is pulled a little extra and a bulge is created. In addition, the other side of the Earth is pulled a little less than the center of the Earth so another bulge is created on the back side. As the Earth rotates about its axis, these bulges move along the surface of the Earth and causes what we call tides. Answer 5: The tidal effect occurs because of the relative closeness of the Earth and Moon. The easiest way to picture the tidal effect is to consider the gravitational effect of the Moon on the earth. In particular consider the differential gravitational tug between a point on the surface of the Earth on the opposite side of the Earth- moon line and a point at its antipode (that is, on the Earth surface directly under the moon). A picture is worth a 1000 words but email media does not allow this. But if you look up tidal effects in almost any geology or astronomy book you will see the picture.

At any rate,the near-side bulge on the earth is a direct consequence of the greater gravitational attraction by the moon compared to the center of the Earth. after all, the near side sub-lunar point is closer to the moon than the center of the earth.. The bulge at the opposite side results from the weaker attraction of the far side Earth point compared to the center... A lot of people have trouble seeing this, but if you draw a diagram with force vectors of the appropriate lengths(qualitatively) and then add the opposite of the force of attraction between the moon and the center of the Earth, you will see that directly below the moon the residual force or the tidal force points directly towards the moon whereas on the opposite side the net force points directly away. The ocean responds almost instantaneously to these forces because the viscosity of water is so low. Answer 6: The moon (or any gravitational body, for that matter) exerts agravitational pull that is stronger the closer you are to it. Since the Earth has a physical size, the gravitational pull on the Earth is stronger on the side facing the moon than it is on the side facing away from the moon.

Because the Earth is being pulled on more strongly on one side than from the other, it is effectively being stretched by the difference in the magnitude of gravitational force from the moon. Most of the Earth, of course, is solid (it is made of rock), but the oceans are not, and, as a result, they are much easier to stretch than the solid Earth. Therefore, the liquid is pulled toward the moon more strongly on the side facing the moon, causing it to accumulate there, while the side opposite the moon, not feeling the moon's pull as strongly, stays on the side opposite the moon. Because the ocean is still held by the Earth's gravity as well, it does not come off the surface, and instead sea level on the side of the Earth facing the moon and on the side opposite the moon rises. At the same time, sea level falls due to a relative lack of water halfway around the Earth from the moon. This change in sea level is what we call tides. Also of note: the Sun has about the same tidal influence on the Earth as the moon does.

If you are good at math, here is the equation that determines gravity: F = G*m*M/(r2), F = Force due to gravity, G = a constant which has to be looked up m = the mass of the moon M = the mass of the Earth r = the distance between the Earth and Moon. The Earth has a diameter of 12,800 kilometers. I think that the moon is about 100,000 kilometers away, though I could be wrong on that. Answer 7: The moon affects the tides depending on its position around the earth. For example, if the sun, the moon and the earth are all lined up in a straight line (180 degree angle) then they form tides call SPRING TIDES (they do not necessarily occur during spring). The Earth's tides are about the same whether the Sun and Moon are lined up on opposite sides of the Earth (full moon phase) or on the same side (new moon phase). During spring tides you will notice higher and/or lower tides than normal.

When the earth, sun and moon form right angles (occurs when the moon is at the first quarter or last quarter phase) to each other they form tides called NEAP TIDES. Neap tides are generally weak tides. Click Here to return to the search form. Copyright © 2015 The Regents of the University of California, All Rights Reserved. The Ocean's Tides Explained Moon Posters The Ocean's Tides Explained The alternating pattern of rising and falling sea level with respect to land is what we know as the tides. What causes this "motion of the ocean"? In one word, gravity. Specifically, the gravitational forces of the Sun and Moon. The key to understanding how the tides work is understanding the relationship between the motion of our planet and the Moon and Sun. As the Earth spins on its own axis, ocean water is kept at equal levels around the planet by the Earth's gravity pulling inward and centrifugal force pushing outward. However, the Moon's gravitational forces are strong enough to disrupt this balance by accelerating the water towards the Moon. This causes the water to 'bulge.' As the Moon orbits our planet and as the Earth rotates, the bulge also moves.

The areas of the Earth where the bulging occurs experience high tide, and the other areas are subject to a low tide. Water on the opposite side of Earth facing away from the Moon also bulges outward (high tide), but for a different and interesting reason: in reality, the Moon and the Earth revolve together around a common gravitational center between them, or center of mass. Here's a rough but helpful analogy: picture yourself swinging a heavy object attached to a rope around your body as you rotate. You have to lean back to compensate, which puts the center of mass between you and the object. With the Earth-Moon system, gravity is like a rope that pulls or keeps the two bodies together, and centrifugal force is what keeps them apart. Because the centrifugal force is greater than the Moon's gravitational pull, ocean water on the opposite side of the Earth bulges outward. The same forces are at play as the Earth revolves around the Sun. The Sun's gravity pulls ocean water toward the Sun, but at the same time, the centrifugal force of the combined Earth-Sun revolution causes water on the opposite side of Earth to bulge away from the Sun.

However, the effect is smaller than the Moon, even given the greater mass of the Sun (greater mass means greater gravitational force). Why? Simply because The Sun is so far away  over 380 times farther away from the Earth than the Moon. Because the tides are influenced by both the Moon and the Sun, it's easy to see that when the Sun lines up with the Moon and the Earth, as during a New Moon or Full Moon (a configuration also called "syzygy"), the tidal effect is increased. These are known as spring tides, named not for the season, but for the fact that the water "springs" higher than normal. On the other hand, if the Sun and the Moon are 90 degrees apart in relation to an observer on Earth as during the First Quarter Moon or Third Quarter Moon (sometimes called half moons), then high tides are not as high as they normally would be. This is because despite its greater distance, the Sun's mass allows it to exert enough gravitational force on the oceans that it can negate some of the effects of the Moon's pull. This phenomenon of lower high tides is called a neap tide.

The height of the tides can also vary during the course of a month because the Moon is not always the same distance from the Earth. As the Moon's orbit brings it in closer proximity to our planet (closest distance within a moon cycle is called perigee), its gravitational forces can increase by almost 50%, and this stronger force leads to high tides. Likewise, when the Moon is farther away from the Earth (furthest distance is called apogee), the tides are not as spectacular. Tides most commonly occur twice a day (diurnal). Tides can also occur as two high waters and two low waters each day (semi-diurnal). However, these periods do not happen at the same time each day. This is because the Moon takes slightly longer than 24 hours to line up again exactly with the same point on the Earth - about 50 minutes more. Therefore, the timing of high tides is staggered throughout the course of a month, with each tide commencing approximately 24 hours and 50 minutes later than the one before it. There are many factors involved in predicting the tides.

In addition to the motion of the Moon and Sun described above, timing of the tides are also affected by the Moon's declination (angular height above the equator), local geography of the coastline, topography of the ocean floor, and depth of the water, among other considerations. Thus, the tides can't be perfectly predicted solely by astronomical calculations that track the Sun and Moon. For greatest accuracy, tide prediction tables always integrate data from actual observation, often over a period of many years. NOAA National Ocean Service Education: Tides and Water Levels Tides and Water Levels Tidal Variations - The Influence of Position and Distance Together, the gravitational effects of the moon and the sun affect the Earths tides on a monthly basis. Click the image to see an animation. The moon is a major influence on the Earths tides, but the sun also generates considerable tidal forces. Solar tides are about half as large as lunar tides and are expressed as a variation of lunar tidal patterns, not as a separate set of tides.

When the sun, moon, and Earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on the lunar tide, creating extra-high high tides, and very low, low tidesboth commonly called spring tides. One week later, when the sun and moon are at right angles to each other, the solar tide partially cancels out the lunar tide and produces moderate tides known as neap tides. During each lunar month, two sets of spring tides and two sets of neap tides occur (Sumich, J.L., 1996). The elliptial orbits of the moon around the Earth and the Earth around the sun have substantial effects on the earths tides. Click the image for a larger view. Just as the angles of the sun, moon and Earth affect tidal heights over the course of a lunar month, so do their distances to one another. Because the moon follows an elliptical path around the Earth, the distance between them varies by about 31,000 miles over the course of a month.

Once a month, when the moon is closest to the Earth (at perigee), tide-generating forces are higher than usual, producing above-average ranges in the tides. About two weeks later, when the moon is farthest from the Earth (at apogee), the lunar tide-raising force is smaller, and the tidal ranges are less than average. A similar situation occurs between the Earth and the sun. When the Earth is closest to the sun (perihelion), which occurs about January 2 of each calendar year, the tidal ranges are enhanced. When the Earth is furthest from the sun (aphelion), around July 2, the tidal ranges are reduced (Sumich, J.L., 1996; Thurman, H.V., 1994). LUNAR PHASES If you watch the moon every night, you see its shape appear to change. Does the moon really change shape? Of course not, but its appearance from Earth certainly changes. How does this work? The answer lies within the part of the moon that receives sunlight, and the part of the moon that does not receive sunlight.

Letï¿½s look at a diagram of the Earth moon system to figure out how this works. Sunlight approaches the Earth-moon system from the right on this diagram (Click on the picture to start the program). Your job is to determine which half of the moon is receives sunlight, and which half of the Earth receives sunlight. Move the mouse on each part of the diagram to arrange the light/dark shades over each moon position, click to set the shadow on the moon. (The program will tell you when you have them all correct). Sunlight will shine on the side of the Earth (or moon) that faces the sun. We call this day on the side of the Earth that faces the sun, and night on the side of the Earth that does not face the sun. Keep in mind that sunlight will illuminate the side of an object that faces it! Click on the arrow in the corner to go to Part 2. OK, so what does this have to do with moon phases? Well, the moon "shines" by reflected sunlight. As the moon orbits Earth, sunlight "shines" on different parts of the moon on the side of the moon that we can see. We always see the same side of the moon. (Why?

Because the moon only spins once per lunar month on its own axis as it orbits the Earth, so we always see the same side.) The key to understanding moon phases is: although the sun always shines on the same side of the moon, the moon is not always in the same place while orbiting Earth with respect to the sun. That sounds confusing, so let's work through a little diagram so you can visually see this and figure it out. Click Activity 2 above to go to the next part of the activity. The diagram still shows the shadows that you correctly identified before, but in this diagram, you will need to determine what the moon looks like to humans on Earth. It's all a matter of perspective. The green dashed line divides the moon into two halves - the half we can see from Earth, and the half we don't see. Look at the moon when it is at position (a). It looks like half of the moon is light and half is dark. Find and click the picture that shows what the moon looks like in position (a). If you pick the correct picture and information box will pop up telling you about the phase. You can close the box and move on to the next phase.

Your job is to continue through all the phases, through (h), to determine if you can figure out which moon phase matches with the position on this diagram. After you get match the picture for all of the phases, Try answering these questions to see if you really understand the facts about the Earth-moon system. Note: you need to have Javascript enabled on your browser to submit your answers. Enter your name: The phases of the moon are produced by: A) the spin of the Earth B) varying amounts of sunlight reaching the side of the moon that faces Earth C) varying amounts of sunlight reaching the side of the moon that faces the sun D) the orbit of the Earth around the moon At full moon: A) the side of the moon facing the Earth receives no sunlight. B) the side of the moon facing the Earth receives full sunlight. C) the moon is between the Earth and the sun D) none of these A) the side of the moon facing the Earth receives no sunlight. B) the side of the moon facing the Earth receives full sunlight.

C) the moon is between the Earth and the sun D) A and C At first and third quarter moon phases: A) the moon is basically at a 90 degree angle from the sun. B) half of the side of the moon we can see "shines" C) A and B D) none of these In the first half of the lunar cycle, the "growing" part of the moon appears on the ______ side. After full moon when the moon is waning, the illuminated part of the moon appears on the ______ side. A) right, left Letï¿½s summarize: At new moon, the moon is nearly in line with the sun. The illuminated part of the moon faces away from Earth, so the moon appears invisible in the night sky. A few days after new moon, the waxing crescent moon appears above the western horizon soon after sunset. Each evening it becomes larger and higher above the horizon. About a week after new moon, the moon reaches first quarter and appears high in the southern sky at sunset. The first quarter moon does not set until about midnight, so to see it you must watch late-night talk shows and go outside after they are finished.

In the days following first quarter, the waxing gibbous moon continues to grow. It appears to move further east, and it sets later in the evening. About two weeks after the new moon, the full moon arrives, rising in the east and setting in the west. The full moon is visible all night long, and it sets in the west around sunrise. The waning phases of the moon are not as visible because the moon is not visible in the early evening when most people are out and about. As the moon wanes through gibbous, it rises later and later. By the time third quarter rolls around, moonrise doesn't occur until around midnight. If you want to see a waning crescent moon, you must get up early before sunrise and look for the moon in the eastern horizon. The lunar cycle (moon phases) repeats with a period of 29.5 days. This is called a synodic period. Synodic comes from the Greek words meaning journey together, referring to the sun and moons positions with respect to each other. Here's a question for you! Can you see the moon sometimes in the daytime?

You'll never see a full moon in the daytime sky because it rises at sunset and sets and sunrise, but you can sometimes see other phases of the moon in the daytime. It looks a little pale and washed out, but it's there. Why doesn't the moon appear as bright in the daytime as it does at night? A) The moon is receiving less sunlight. B) The sunlight in the atmosphere makes the moon appear less bright. C) The moon does not reflect light in the daytime. Here's another question for you! Let's see if your brain is really on today! If you can see a full moon at your position on the Earth, what type of moon will you see if you go to exactly the opposite side of the world on the same day (24 hours)? A) A new moon. C) No moon at all. D) None of these. If you see two full moons in the same month of a calendar year, the second full moon is called a: A) Moon River B) Blue Moon C) Moonraker One last point: Remember that everyone on the planet will see the same phase of the moon at the same time. It is the Earth that rotates, giving us night and day.

Further thought question: We know that tidal effect occurs primarily because of the moon orbiting the earth. The Earth's rotation is what causes the effect of two high and two low tides per day and each location. Do you think there would be a correlation between the time of moonrise and the time of high tide? Why or why not? This part puts everything in action and lets you observe moon moving around the earth in it's orbit. You can click on the moon and drag it with the mouse. Click the "Start Simulation" button and the moon will move in it's orbit and the earth will spin on it's axis. (The timing of the simulation is scaled accuratly, The earth makes a complete rotation 29.5 times in the time it takes the moon to go through a complete cycle). You can also click on the number buttons to jump to a certain day in the moon's phase cycle. Or click on the phase buttons 'First Quarter', 'New Moon', etc.

Question: What kind of tide appears at full Moon?
Answer: Spring tide